1. Field of the Invention
The present invention relates to techniques of making it difficult to illegally copy data stored in a memory device.
2. Description of the Background Art
Various kinds of games have conventionally been provided in the form of game cartridges to home game machines. A typical game cartridge incorporates a mask ROM that stores information with wired logic, and stores application software and the like for implementing a prescribed game in the mask ROM. A wide variety of games are to be enjoyed by changing the game cartridge inserted in a game machine.
This type of game cartridges are vulnerable to illegal copying.
One method of reading data in a game cartridge is to analyze input and output signals to and from the game cartridge.
To prevent reading of data by such analyzing method, Japanese Patent Application Laid-Open No. 2002-091828 discloses a technique of encrypting input signals to a game cartridge and output signals from the game cartridge.
Encrypting input or output signals to or from a game cartridge as mentioned above requires key information about encryption or decryption to be stored beforehand in the game cartridge.
However, when the key information is stored in a mask ROM that stores information with wired logic in the cartridge, there are fears that the key information will be analyzed through analysis of a circuit structure of the mask ROM.